The present invention relates generally to transformers, and in particular to a system and method of controlling saturation of magnetic cores of bi-directionally driven transformers.
Transformers, such as those utilized in DC-DC converters for switching power supplies, often include magnetic cores. These magnetic cores store a magnetic field based upon the field generated by current flowing through the primary winding(s) of the transformer. The generated field is dependent upon the number of turns and the core cross-sectional area of the transformer, as well as the magnitude of current flowing through the transformer. Magnetic saturation may occur within the core when the generated field is no longer capable of further increasing the magnetization of the core. This results in the output voltage of the transformer falling to zero, as well as overheating of the transformer.
In systems such as DC-DC converters, bi-directional current is often provided to excite the transformer. In past systems, saturation of the magnetic core has been detected by sensing the primary current of the transformer and comparing the sensed current with a saturation threshold. However, the use of a current sensor or sense resistor is limited in that it is only capable of detecting a transformer output indicative of saturation based upon a perceived saturation threshold.
Operating regions of magnetic cores, as illustrated in hysteresis charts (“BH loops”), include both linear and non-linear regions. Magnetic cores operate in the linear region up until a “knee-point” of the BH loop for the magnetic core. Following the “knee-point,” magnetization of the core changes at a non-linear rate and moves into saturation. Due to temperature effects on permeability, core volume (tolerances of core size), variation in manufacturing and other external tolerances (i.e., tolerances of a current sensor), a saturation threshold has been selected conservatively to ensure it remains within the linear range. Because the output current level of the transformer is not indicative of an operating point of the magnetic core, controls implemented based upon the current sensor may lead to problems such as, for example, direct current offsets within the magnetic core which reduce the operating range of the transformer.